Cellulose is a renewable natural polymer that can be converted to many chemical derivatives. The derivatization takes place mostly by chemical reactions of the hydroxyl groups in the β-D-glucopyranose units of the polymer. By chemical derivatization the properties of the cellulose can be altered in comparison to the original chemical form while retaining the polymeric structure.
If cellulose in fibers is derivatized in a suitable way, the fibers are easier to disintegrate to the level of fibrils, nanofibrillar cellulose, because of weakened bonds between the fibrils. For this purpose the cellulose can be anionized or cationized. For example catalytic oxidation of cellulose by heterocyclic nitroxyl compounds (such as “TEMPO”, i.e. 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical) produces anionic cellulose where part of C-6 hydroxyl groups are oxidized to aldehydes and carboxylic acids. Another method to produce anionic cellulose is carboxymethylation of cellulose molecules. Cationic cellulose can be produced by adding quaternary ammonium groups to cellulose molecules.
In practice, pulp which contains cellulosic fibers in suspension is subjected to chemical modification to reach a suitable degree of substitution, whereafter the fibers are disintegrated to fibrils with nanofibrillar cellulose as product.
Nanofibrillar cellulose can be produced in a variety of ways, but the common feature is that the modified pulp is processed at a relatively low consistency. Consequently, the resulting nanofibrillar cellulose is a liquid dispersion with a correspondingly low concentration. The concentration of the nanofibrillar cellulose in the dispersion is usually below 5 wt-%, usually about 1 to 4 wt-%.
One of the most prominent physical properties of the nanofibrillar cellulose is that it forms a highly viscous gel in concentrations above 1%. Raising the concentration of this type of gel to decrease transportation costs from the manufacturing location is desirable. Although methods have been developed for lowering the water content of the gel, it requires time and energy, and increase the price of the nanofibrillar cellulose. With some grades of nanofibrillar cellulose, excessive dewatering or drying can even alter the properties of the nanofibrillar cellulose so that it has no longer the same rheological characteristics when it is redispersed in water at the location of use.